Capacitance: Help me understand

Maybe since I managed to double post this I should amend my comments above to read I am "stupid and clueless"

Capacitance is the capacity (get it?) of your wire to store electrical charge (i.e. the energy coming from your amp on its way to your speakers). If your speaker cable has lots of (i.e., high) capacitance, they will store energy as it enters the wire (when any loud passage occurs in the music) and dicharge it as soon as the level drops (when load pasage is over). This is bad for two reasons: 1. When your cable is fully charged, it tends to block (impede) signals trying to get trough. This can only muck-up the sound. 2. When music level drops, the stored up energy will discharge into your speaker creating a signal that wasn't on your CD (or whatever) creating "ghosting" (like a bad T.V. picture before cable T.V.). This will blur the sound and cause other unmusical havoc. Capacitance is caused by lots of things like thickness and shape of the wire and what the wire is covered with. Teflon is used in high-end designs because it doesn't like electricity and will store almost none (low capacitance). Enjoy our hobby!

High capacitance usually rolls off the high frequencies, not the midrange -- plus, I didn't think the WW Gold Eclipse III was a high-capacitance design. Maybe something else is responsible for the reticent midrange; without more details, I couldn't say. Good luck, let us know if you track down the problem. One cheap way to see if it's the cables is to swap the WW cables for something different and see if the balances changes to your liking -- heck, some standard 14 or 16 gauge zip cord from the hardware store would tell you what you need to know.

There are few errors in kkirkpa's post. Yes, it has to do with the storage of electrical charge. But the idea of storing energy during loud passages and releasing energy during soft passages is a little off base. The signal from the pre-amp is constantly charging and disharging this capacitance. The energy is stored and released at the same frequency as the electrical signal, i.e. if fed with a 1000 cycle signal, the cable is being charged and discharged at this rate. The preamp sees this capacitance as part of the total load it must drive. Loudness really has nothing to do with it. It turns out that the higher frequencies are more affected by this than the lower. The higher frequency currents flow more easily through the capacitance than the low frequencies, so as Plato points out, you have less high frequency energy reaching the speakers as the capacitance increases. Some very wide bandwidth amplifiers like Spectral recommend using high capacity interconnects from the preamp to limit the very high frequencies getting into the amp.
The construction of the cable does determine this capacitance. There are three factors. Size, distance, and dielectric. Larger wires with more surface area and putting the wires closer together will increase the capacitance. Changing the dielectric (the material separating the wires like the teflon mentioned above) will also affect it. Air has the lowest dielectric constant and will give the least amount of capacitance.
That being said, I doubt that this is your problem. I would suggest experimenting with speaker placement as a way to affect your center image and soundstage. Start by moving them close together and toed in until you get a clearly defined center image with limited soundstage. Then move them apart and toe them out to expand the soundstage until your center image gets too diffuse. Somewhere in between is where you want to be. There will be a sweet spot where you get both. Of course, this also affects the frequency balance. It takes a lot of time a patience to get it right, but nobody said this was going to be easy.